Method and device for controlled straightening and cooling of a wide metal strip, especially a steel strip or sheet metal, running out of a hot rolled strip rolling mill

ABSTRACT

A method and device for controlled straightening and cooling of a wide metal strip, especially a steel strip or sheet metal, running out of a hot rolled strip rolling mill, using pinching rollers arranged in the moving direction of the strip behind vertical double rollers, said pinching rollers producing a tensile stress acting in a longitudinal direction. According to the invention, the range of use of conventional sheet metal cooling systems can be extended to obtain a more even surface of said steel strip with an increased cooling effect. This is achieved by displacing the metal strip or sheet metal between a pre-straightening machine and splash cooling facility in defined conditions of tensile stress by adjusting the tensile stress and by cooling said strip or sheet metal inside said splash cooling facility between successive pairs of pinching rollers and by additionally controlling the tensile stress.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application of U.S. patentapplication Ser. No. 10/498,652, filed Jun. 10, 2004, which is a 371 ofInternational application PCT/EP02/013035, filed Nov. 21, 2002, whichclaims priority of DE 101 63 070.0, filed Dec. 20, 2001, the priority ofthese applications is hereby claimed and these applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention concerns a method and device for controlled straighteningand cooling of wide metal strip, especially steel strip or sheet,running out of a hot-rolled strip mill, with the use of pinch rolls,which are installed after (in the direction of strip flow) verticaldouble rolls and produce a tensile stress that acts in the longitudinaldirection.

The method and device described above are basically known from DE 33 13024 A1, which discloses a method and a device for quenching passingsteel sheet, especially heavy and medium sheet, with simultaneouscontrolled straightening, in which the sheet, before being quenched,straightened in a plane while hot by at least two vertical double rollsarranged one behind the other. The sheet is then drawn through aquenching installation under controlled tensile stress by at least onedouble roll. This method is designed to quench passing steel sheetexactly to the predetermined mechanical properties without thedevelopment of warpage or distortion and at the same to reduce roughnessthat is already present. Despite these measures, inadequate flatness ofthe sheet continues to occur due to inadequate application of tensilestress, unfavorable cooling, and other unfavorable control measures. Thecooling devices used in practice, which consist essentially of nozzleunits or, for the aftercooling, of laminar water curtains, cannot reallybe improved any further. In principle, a U-tube is used for laminarcooling units. Of course, the highest attainable cooling intensities areobtained with nozzles and suitably selected parameters. The poorautomatic control and regulation behavior is a disadvantage in generaland at low flow rates in particular.

SUMMARY OF THE INVENTION

The objective of the invention is to extend the range of application ofconventional sheet cooling systems to obtain greater flatness of thesheet with an increased cooling effect.

In accordance with the invention, this objective is achieved by movingthe metal strip or sheet between a pre-straightening machine and asplash cooling installation under defined conditions of tensile stressby adjustment of the tensile stress and by cooling the metal strip orsheet inside the splash cooling installation between successive pairs ofpinch rolls and additionally controlling the tensile stress.

This directly produces plastic deformation before the metal strip orsheet passes into the cooling installation. The plastic deformationimproves the flatness of the entering metal strip or sheet. Anotheradvantage is gained by the arrangement of the straightening processimmediately before the cooling, which prevents renewed buildup ofinternal stresses and deformations due to the nonuniform cooling on thesurface of the metal strip or sheet.

The cooling effect is practically limited by the induction of stressesand deformations associated with these stresses in the sheet due to thenonuniform cooling effect of the cooling medium (water) on the surfaceof the sheet. This (unfavorable) effect occurs especially with the useof a relatively high cooling intensity on sheets that are thin and atthe same time wide. The critical product of a plant is defined by asheet with the least thickness at the maximum width and at the same timehigh intensity of the cooling. The intensity of the cooling is definedby a high temperature difference between the initial and final coolingtemperature using a short cooling time. In practice, pinch rolls areintended to hinder this deformation effect. However, the effect of thisform of compensation is limited. The arrangement of thepre-straightening machine before the cooling installation in accordancewith the invention brings about plastic deformation and reduces entrydeformations and stresses directly before the cooling installation.

The deformations on entry into the cooling installation can be combatedespecially effectively by adjusting the tensile stress by switching thespeed to the respective next or last supporting pinch roll withprogressive passage of the metal strip or sheet.

In one embodiment, the pinch rolls are set with a spacing such that acooling unit is formed between two rolls, depending on the spacing.

In a refinement of the invention, this spacing can be adjusted to ahalf-length of the resulting characteristic form for a strip or sheetwith the least thickness, at maximum width and high cooling intensity.

The device for the controlled straightening and cooling of a wide metalstrip, especially a steel strip or sheet, running out of a hot-rolledstrip mill is equipped with vertical pairs of double rolls and pairs ofpinch rolls which follow them (in the direction of strip flow), betweeneach of which a cooling unit is installed.

In accordance with the invention, the stated objective is achieved witha device of this type by installing a pre-straightening machine beforethe pairs of pinch rolls with the cooling units, between which acontrolled tensile stress can be produced. This makes it possible totransmit a tensile force even before the cooling, so that the magnitudeof the deformations that are present is already reduced here. An evenmore exact determination of the tensile stresses inside the coolingunits then occurs.

In accordance with other features, the cooling units are each designedfor a high and an intermediate cooling intensity.

The effect of the cooling can be increased still further by providingthe cooling unit with finely adjustable cooling elements at the outletof the metal strip.

The cooling can be further developed in such a way that the cooling unitis designed for high and/or an intermediate cooling intensity inaccordance with a predetermined product spectrum.

The invention can be further improved by also making it possible toproduce tensile stresses between the last, supporting pinch rolls or thestraightening rolls at the trailing end of the metal strip or sheet andthe pinch rolls or the straightening rolls at the leading end of themetal strip or sheet.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of the disclosure. For a better understanding of the invention, itsoperating advantages, specific objects attained by its use, referenceshould be had to the drawings and descriptive matter in which there areillustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 shows a system diagram of the cooling with pinch rolls and of thedrive control with the tensile stresses produced.

FIG. 2A shows a perspective view of the steel strip or sheet withdistortions in the form of center waviness.

FIG. 2B shows a side view of two pairs of pinch rolls arranged with acertain spacing distance.

FIG. 3 shows the arrangement of the cooling devices for the givencooling intensity.

FIG. 4 shows a view of a total installation for straightening andcooling.

DETAILED DESCRIPTION OF THE INVENTION

The metal strip, especially the steel strip 1 or sheet, is conveyed by atensile stress 6 acting in the longitudinal direction (strip flowdirection 2) for controlled cooling and straightening with the use ofpinch rolls 5 arranged after (in the direction of strip flow 2) verticaldouble rolls 3 and 4. In the process, the metal strip 1 is moved betweena pre-straightening machine 7 and a splash cooling installation 8 underdefined conditions of tensile stress 6 by adjustment of the tensilestress (by the straightening roll rotational drive motors). The metalstrip 1 is then cooled inside the splash cooling installation betweensuccessive pairs of pinch rolls 5 a, and the tensile stress isadditionally controlled. The splash cooling installation 8 comprisescooling units 9 installed between each two pairs of pinch rolls 5 a(FIG. 1). While the metal strip 1 is entering at roller table speed 10,the tensile stress 6 is controlled via a suitable torque 11 by afeedback control system 12, which is designated (A) action of the pinchrolls (i) to (i+nc). As soon as the trailing end 1 a of the strip passesthrough, which is designated (B) action of the pinch rolls 5 (i) to(i+nc), suitable tensile stress is transmitted to the trailing end 1 aof the strip, and only the comparison speed is still measured beyond theend 1 a of the strip without using the feedback control system 12. Bothfor the trailing end 1 a of the strip and the leading end 1 b of thestrip, the tensile stress 6 is adjusted by switching the speed to therespective next or last supporting pinch roll 5 with progressive passageof the metal strip (see FIG. 1).

The pinch rolls 5 are installed with a certain spacing 13, such that thecooling unit 9 is formed between two pinch rolls 5, depending on thespacing 13. For example, the spacing 13 can be based on a half-length 14of the resulting wavelength 15 for a metal strip with the leastthickness, at maximum width and high cooling intensity. The typicalwavelength 15 develops in the steel strip 11 (FIG. 2A).

The cooling units are each designed for a high or an intermediatecooling intensity (FIG. 2B), such that the spacing 13 corresponds to thehalf wavelength 15 (cf. FIG. 2B). The behavior of the wave 16 is clearlyshown by the dotted lines 17 in FIGS. 2A and 2B.

FIG. 3 shows the cooling effect of the different individual types ofcooling devices 9. In the graph of temperature as a function of time,the solid curve represents the behavior of the temperature 20 in thecenter of the strip or sheet, and the broken curve represents thesurface temperature 21. Splash cooling units 8 a are used in thesections (1). Cooling units 9, which consist of U-tube units 22, areprovided at the outlet 18 of the steel strip 1.

On the basis of the temperature ΔT-center (1), the temperature in thecenter of the steel strip 1 or the temperature ΔT-center (2) is reached.

The temperature ΔT-surface is reached while still in the zone of thesplash cooling units 8 a. When the U-tube units 22 are used, thenecessary cooling stop temperature is attained in small increments bythe U-tube units 22. Martensite formation by surface subcooling isprevented above the temperature T-martensite.

Tensile stresses 6 can be produced in the same procedure between thelast supporting pinch rolls 5 or straightening rolls 23 at the trailingend 1 a of the metal strip 1 or sheet and the pinch rolls 5 or thestraightening rolls 24 at the leading end 1 b of the metal strip 1.

FIG. 4 shows a complete installation, in which, in the direction ofstrip flow 2, the pre-straightening machine 7 is followed by the splashcooling installation 8 with the pairs of pinch rolls 5 a and the coolingunits 9 and then by a laminar cooling installation 25.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the inventive principles, it will beunderstood that the invention may be embodied otherwise withoutdeparting from such principles.

1. Method for controlled straightening and cooling of wide metal stripor sheet running out of a hot-rolled strip mill, with the use of pinchrolls, which are installed after (in the direction of strip flow)vertical double rolls and produce a tensile stress that acts in thelongitudinal direction, wherein the metal strip or sheet is moved from apre-straightening machine immediately to a splash cooling installationunder defined conditions of tensile stress by adjustment of the tensilestress, and that the metal strip or sheet is cooled inside the splashcooling installation between successive pairs of pinch rolls, and thetensile stress is additionally controlled, whereby the metal strip orsheet is plastically deformed prior to entering the coolinginstallation.
 2. Method according to claim 1, wherein the tensile stressis regulated by switching over the speed with respect to each next, lastsupporting pinching roller with progressive transit of the metal stripor of the sheet.
 3. Method according to claim 1, wherein the pinchingrollers are fixed at a spacing, wherein a cooling unit is formed betweentwo pinching rollers depending on the spacing.
 4. Method according toclaim 3, wherein the spacing is adjusted to a half-length of a resultingintrinsic shape for a metal strip or a sheet with smallest thickness atmaximum width and high cooling intensity.
 5. Device for controlledstraightening and cooling of a wide metal strip or sheet running out ofa hot-rolled strip mill, with vertical pairs of double rolls followed(in the direction of strip flow) by pairs of pinch rolls, between eachof which a cooling unit is installed, wherein a pre-straighteningmachine is installed immediately before the pairs of pinch rolls withthe cooling units, between which a controllable tensile stress can beproduced, whereby the metal strip or sheet is plastically deformed priorto entering the cooling installation, wherein the metal strip or sheetis moved from a pre-straightening machine immediately to the coolingunits under defined conditions of tensile stress by adjustment of thetensile stress.
 6. Device according to claim 5, wherein the cooling unithas finely adjustable cooling elements and is provided in an outlet forthe metal strip.
 7. Device according to claim 5, wherein the coolingunit is designed for a high and/or medium cooling output in accordancewith a predetermined product spectrum.
 8. Device according to claim 5,wherein tensile stresses can also be produced between the last,supporting pinch roll or the straightening roll at the trailing end ofthe metal strip or of the sheet and the pinch rollers or thestraightening rollers at the leading end of the metal strip or sheet. 9.Device in accordance with claim 5, wherein the cooling units are eachdesigned for a high and an intermediate cooling intensity.